The invention is related to the field of photodiodes, and in particular to dark current reduction for large area photodiodes.
Large area, high performance photodiodes are critical for advances in information technology (polymer optical fiber (POF) networks, multimode optical fibers), for advanced photovoltaic applications and for imaging applications. Because of low cost requirements, high performance semiconductor materials are increasingly deposited on low cost substrates like silicon.
The epitaxial growth of lattice mismatched semiconductor materials for devices generally results in misfit and threading dislocations as well as point defects in the epitaxial grown semiconductor material. The high defect density leads to poor device performance, generally resulting in a large dark current of the device. An example is epitaxial germanium (Ge) on silicon (Si). The lattice mismatch between Si and Ge is 4.2% and Ge devices like photodiodes show large dark current for large area devices (>50 μm diameter). Similarly, large dark current due to lattice mismatch has for example limited device performance for CdZnTe on Si or HgCdTe on Si. Similar results can also be observed for SiGe alloys and III-V semiconductor materials, epitaxially grown on Si.
Several methods have been developed in the past to alleviate the dark current problem. For small size devices, etched mesas combined with high temperature annealing helped reduce threading dislocation densities and point defects, resulting in smaller dark currents. Another approach was the introduction of graded buffer layers, epitaxial layers of materials with smaller lattice mismatch, that reduce the threading dislocation density. While this method can reduce dark current significantly, the several micron thick buffer layer is in many cases undesirable.
Recently it has been shown that micron sized Ge detectors, grown selectively in windows of a dielectric material like silicon oxide or silicon nitride on Si have very low dark currents. Two effects lead to the small dark current. First, the small size allows threading dislocation reduction very similar to the mesa approach described before. Secondly, the growth in the dielectric window improves the sidewall passivation of the detector significantly.
Large detectors of lattice mismatched materials can utilize these effects to reduce dark currents by dividing the large area into smaller sections that are separated by a dielectric material, thereby reducing misfit dislocations due to the mesa effect and improving side wall passivation due to the growth in the dielectric window.